You can have extremist atheists... but not about and because of their atheism 
I think atheism without critical thought is less likely to lead to something like, say, 911 than theism (Islam in this case). Because it would need more than just lack of critical thought to go that bad... atheism would also need a gospel (or something to that effect) that was completely unquestioned by atheists... and the content of it would have to cause nasty consequences (to say the least) if so zealously believed in.
EvF
I think atheism without critical thought is less likely to lead to something like, say, 911 than theism (Islam in this case). Because it would need more than just lack of critical thought to go that bad... atheism would also need a gospel (or something to that effect) that was completely unquestioned by atheists... and the content of it would have to cause nasty consequences (to say the least) if so zealously believed in.
EvF
